U.S. Pat. Appl. No. 2008/0047964 (Denner et al, assigned to CO2PAC) discloses a container comprising a pressure panel located in the bottom portion of the container.
According to Denner, the pressure panel is movable between an outwardly-inclined position and an inwardly-inclined position to compensate for a change of pressure inside the container. In order to alleviate all or a portion of the vacuum forces within the container, the pressure panel is moved from the outwardly-inclined position by a mechanical pusher after the container has been capped and cooled, in order to force the pressure panel into the inwardly-inclined position.
Tests conducted on such a container showed that, once inverted to the inwardly-inclined position, the pressure panel does not maintain its position but tends to sink back under the pressure of the content. In the end, after the content has cooled, the container has lost much rigidity and therefore feels soft when held in hand. When stacking or palletizing the containers, there is a risk for the lower containers to bend under the weight of upper containers, and hence a risk for the whole pallet to collapse.
This design was enhanced by European Pat. Appl. No. EP 2 957 522 (Sidel Participations), which proposes a container including an invertible diaphragm having an inwardly-inclined position, wherein it has an inner portion, with a concavity turned outwards with respect to the container, and an outer portion, with a concavity turned inwards with respect to the container.
In order to achieve this configuration, in an outwardly-inclined position the diaphragm is curved in radial section with a concavity turned outwards with respect to the container. It is forced inwards by means of a mechanical pusher.
Although this design is better than Denner's for it provides more rigidity to the container, it still needs to be enhanced since, in some cases, the diaphragm cannot withstand the pressure inside the container and collapses back to its outwardly-inclined position.
Tests conducted with the container at least partly explain this behavior:                the extraction volume (i.e. the volume swept by the diaphragm between its outwardly-inclined and its inwardly-inclined position) is insufficient;        weakness of the diaphragm leading to its collapse seems to be located at the junction between the inner portion and the outer portion;        the shape of the diaphragm makes it difficult to achieve inversion and hence to reach the inwardly-inclined position when the filling level of the container is high or the filling product too hot.        